1. Field of the Invention
This invention relates to apparatus and techniques for improving the automatic generation of lathed contact lenses and more particularly for controlling the center thickness of lathed lenses.
2. Brief Description of the Prior Art
Heretofore, the techniques of lathing surfaces, and particularly convex surfaces, of contact lenses were substantially manually performed. In each instance it was necessary that a highly skilled technician perform the operation. Much of the operation was subject to and dependent upon the manual dexterity, coordination, vision perceptiveness and artistic abilities of a contact lens machine lathe operator.
The prior principal manual techniques employed are hereinafter described. One was generally referred to as the "disappearing dot" technique. After a contact lens button had its base or concave surface generated, it was blocked in preparation for the machining of the convex surface. The relatively flat, but rough, convex surface was marked with a crayon type marker. It was then necessary for the operator to present the cutting edge of the pivotal cutting tool to the button such that the cutting tool intersects and begins generating a finished convex curve surface starting first at the outside periphery of the cylindrical button. Since this generation takes place about a cutting tool pivot point in an arcuate path, it will be appreciated that as the cutting tool is traversed inward toward the pivot point and caused to penetrate the body of the button farther and farther, the crayoned front surface becomes smaller and smaller until a circular crayoned dot exists on the convex surface concentric with the base curve. At the instant that the dot is machined away, thus the name "disappearing dot" technique, the operator reads the machine gauges to determine the measurement of the rough center thickness of the contact lens from the apex of the base curve to the apex of the convex surface. This registers the apex of the convex surface relative to the cutting tool pivot point. From that measurement the operator is then able to determine, based upon the desired final center thickness, how much more penetration of the cutting tool is necessary in order to generate the desired final convex surface and center thickness.
As can be appreciated, it is necessary for the operator, usually under magnification, to identify precisely when the crayon mark of the center dot is finally lathed away or disappears so that registration of the cutter relative to the base curve can be made. This is a very tedious process. Typically, in this type of machining, final lens tolerances can stack up as much as .+-.0.04 millimeters. Further, the operation is time consuming, taking as much as 2 minutes to perform the cutting of the front surface.
Another manual technique employed is a variation of the "disappearing dot" technique. In this procedure the tool is parked on the workpiece spindle centerline and the spindle holding the workpiece is manually fed toward the tool tip until a first touch or cut of lathe material is observed. Essentially, the operator must use his vision, aided by a magnifier, to determine the position of that "first cut" of the tool tip. In essence, this procedure is similar to the "disappearing dot" technique, however, a minute step beyond the disappearance of the dot is employed. After that minute step and the first noticeable lathe cut of the button, registration of the button relative to the pivot point of the tool is accomplished and a predetermined radius can then be applied for generation of the convex surface of the contact lens. This procedure is time consuming, taking the skilled operator approximately 20 or more seconds to accomplish and provides a repeatability of tolerance stack-up, for this step alone, in the vicinity of .+-.0.02 millimeters.
Another technique very similar to the previously described "first cut" technique involves parking a mounted dial indicator probe on the spindle centerline and manually bringing the rough lens face into a position of initial or first contact with the probe of the dial indicator. Reading of the dial allows the operator to provide for registration of the contact lens button with respect to the cutting tool. This technique is no more accurate or repeatable than the previous technique where the cutting tool is parked on the spindle centerline.
Another principal manual technique is referred to as "geometric blocking". A solid cylindrically shaped contact lens material button is secured, for example by adhesive, at one circular flat end to a finished blocking platform, shaped like a plug. The blocking platform or block has a uniformly cylindrical outside surface with two substantially parallel end surfaces, to one of which surfaces the flat end surface of the button is secured. The securing process registers the exposed or other end surface of the lens button to the exposed or opposite end surface of the block. After the button is secured to a selected or first surface of the block, the block at its exposed end is chucked within a lathe collett and a base or concave curve is generated into the exposed end portion of the button.
The combination of the block and the button with the generated surface, is removed from the collett and inserted into a blocking machine for blocking of the concave surface to a mating head of a second block for registration of the button with a surface of the second block. This blocking is carried out such that the mating surface of the second block is concentrically mounted to the generated concave surface. This accomplished, the button is captured between the two respective blocks. Subsequently, the cylindrical first block is removed from that face of the button to which it is adhesively attached and which is yet to undergo surface generation. In this manner the reference surface is transferred from the first block to the base of the second block.
The last surfacing operation involves machining a convex surface. After the first block has been removed, the second block is chucked within a lathe collett and machining of the convex surface is carried out relative to the base of the second block. Therefore, the ultimate center thickness of the contact lens is controlled in this last machining step.
This operation has slightly less stack-up tolerances than does the "disappearing dot" technique. Following the "geometric blocking" technique, the stack-up tolerances are approximately .+-.0.03 millimeters. This procedure takes slightly less time than does the "disappearing dot" process.